1. Field of the Invention
This invention relates to an electrophotographic apparatus such as an electrostatic image transfer-type color copying machine and a laser printer.
2. Description of the Prior Art
A conventional electrostatic image transfer-type color copying machine is provided with a developer unit disposed adjacent to the surface of a photoconductor, the developer unit having a magnetic developing roller used to apply toner to the photoconductor to convert a latent image formed on the photoconductor into a visible image. Generally, the developer unit has a shutter which is opened and closed to control the distribution of the toner to the surface of the magnetic developing roller that faces the electrostatic latent image areas on the photoconductor, thereby controlling the distribution of the toner to the photoconductor. Furthermore, in such a copying machine, it is usual to apply a developing bias voltage to the magnetic developing roller of the developer unit during the copy process in order to prevent adherence of the toner to the non-image areas where no image is formed on the photoconductor. The developing bias voltage is maintained at a constant level throughout the copy process. Japanese Patent Publication No. 55-42392 discloses a copying machine wherein a lower voltage than a developing bias voltage is applied when the photoconductor is cleaned with the magnetic developing roller, but in this case also, the developing bias voltage is maintained at a constant level, although bias voltage is varied for the cleaning process.
However, when the developing bias voltage is maintained at a constant level throughout the copy process as described above, the following problems arise when the shutter is in a closed position, while there is not much problem when the shutter is in an open position. That is, since the magnetic developing roller is sparsely loaded with developer after the shutter of the developer unit is closed, the carrier tends to stick to the photoconductor. If carrier sticks to the photoconductor, toner will not adhere to the areas on the photoconductor where the carrier has stuck, possibly causing blank spots in the image transferred to the transfer paper, or if blank spots are not caused, resulting in an incomplete transfer. This also creates a problem in that the surface of a fuser roller is damaged by the carrier stuck on the copy paper.